In recent years, the public attention has been focused on a liquid crystal display because it is capable of replacing the Braun tube since it can be designed to be as thin as desired and it can be operated with a minimum consumption of power. In view of this, in order to improve the functions of such liquid crystal display, polycrystalline silicon thin film transisters (hereinafter referred to as "polycrystalline silicon TFT") have been highlighted and various studies have been made thereon.
The studies on the polycrystalline silicon TFT have been made with the view of effectively forming a polycrystalline silicon semiconductor film on a commercially available insulating substrate such as a soda-lime glass at a low deposition temperature. However, at the present time, an industrially applicable film-forming process which makes it possible to stably and repeatedly form a high quality polycrystalline semiconductor film on such commercially valiable insulating substrate has not yet been realized.
Incidentally, a number of proposals have been made with respect to the plasma chemical vapor deposition process utilizing RF glow discharge, which is generally known as RF glow discharge decomposition process. In accordance with the RF glow discharge decomposition process, it is possible to form a polycrystalline semi-conductor film on an insulating substrate at a relatively high deposition temperature by means of RF glow discharge in a raw material gas to decompose said raw material gas and produce plasma causing film deposition.
However, there are disadvantages for the RF glow discharge decomposition process that the utilization efficiency of a film-forming raw material gas is not satisfactory; there exist a number of film-forming parameters which are organically interrelated with each other and it is extremely difficult to generalize them; and thus, it is difficult to stably and repeatedly obtain a desirable polycrystalline semiconductor film with a high yield.
In order to eliminate these disadvantages for the RF glow discharge decomposition process, attention has been focused on the microwave plasma vapor deposition process (MW-PCVD process) using a microwave power instead of the RF power (high frequency power) wherein a raw material gas is decomposed with the action of a microwave energy to produce plasma causing the formation of a deposited film on a substrate. It is possible to form a polycrystalline semiconductor film on an insulating substrate by the MW-PCVD process. For the MW-PCVD process, there are advantages that plasma causing the formation of a deposited film is produced with a higher density and a film is formed with a higher deposition rate respectively in comparison with those in the case of the RF glow discharge decomposition process. However, as in the case of the RF glow discharge decomposition process, there are disadvantages for the MW-PCVD process. That is, there exist a number of film-forming parameters which are organically interrelated with each other and it is extremely difficult to generalize them, and it is difficult to stably and repeatedly obtain a desirable polycrystalline semi-conductor film with a high yield.
In order to improve the above MW-PCVD process, there has been proposed an electron cyclotron resonance plasma chemical vapor deposition process (hereinafter referred to as "ECR plasma CVD process") which comprises applying magnetic field in the MW-PCVD process. For the ECR plasma CVD process, it has been reported that it is possible to form a polycrystalline semiconductor film with reduced defects more efficiently, at a lower deposition temperature and at an improved deposition rate in comparison with the case of the MW-PCVD process. However, in the factual situation for the ECR plasma CVD process, there are still unsolved problems that in order to obtain a desirable polycrystal semiconductor film, it is required to properly adjust not only the conditions for generating plasma from a raw material gas with the action of a microwave energy but also the conditions for controlling ion energy in the plasma by the application of a magnetic field; it is difficult to properly adjust the foregoing two kinds of conditions in terms of organic interrelation to be in a desired state which allows the formation of such desirable polycrystalline semiconductor film; and because of this, it is difficult to stably and repeatedly obtain a desirable polycrystalline semiconductor film.
An example of the ECR plasma CVD process is described in Column 31P-K-2 of Advance Summary for 1985 Spring Meeting of Applied Physics Society. This literature discloses an ECR plasma CVD process for forming a polycrystalline silicon film or a single crystal silicon film on a single crystal silicon wafer using the reactive ion beam deposition (RIBD) apparatus having the constitution shown in FIG. 6. The apparatus shown in FIG. 6 comprises a plasma generation chamber 601 having a plasma generation space and a deposition chamber 602 having a film-forming space. The upper wall of the plasma generation chamber 601 is hermetically provided with a microwave introducing window 603 connected to a waveguide 604 extending from a microwave power source (not shown). Numeral reference 612 stands for a lower wall of the plasma generation chamber 601 which is constituted by an insulating member and which has a hole through which plasma generated in the plasma generation space of the plasma generation chamber 601 passes into the film-forming space of the deposition chamber 602.
Numeral reference 611 stands for a grid electrode comprising a metal mesh plate for applying an electric field which is placed on the lower wall 612 such that the hole of the lower wall is apparently sealed thereby.
Numeral reference 605 stands for a gas feed pipe extending from a gas reservoir (not shown) in which a raw material gas is contained. Numeral reference 610 stands for a cooling unit provided with the outer wall of the plasma generation chamber 601.
Numeral reference 610' stands for a pipe for supplying a cooling water into the cooling unit 610 and numeral reference 610" stands for a pipe for recycling the cooling water from the cooling unit 610. Numeral reference 609 stands for a magnet which is so provided as to surround the plasma generation chamber 601.
The deposition chamber 602 has a hole with its upper wall through which plasma generated in the plasma generation space of the plasma generation chamber 601 is allowed to pass into the film-forming space of the deposition chamber 602. The deposition chamber 602 is provided with an exhaust pipe connected through an exhaust valve to a vacuum pump (this part is not shown). Numeral reference 613 stands for a conductive substrate comprising a single crystal silicon wafer placed on a conductive substrate holder 608. Numeral reference 607 stands for a flow of the plasma from the plasma generation space. Numeral reference 600 stands for a D.C. power source electrically connected to the grid electrode 611. The power source 600 is electrically connected also to the substrate holder 608 while being electrically grounded.
The process to be practiced by the use of the apparatus shown in FIG. 6 which is described in the foregoing literature is to form a polycrystalline silicon film on the single crystal silicon wafer 613 placed on the conductive substrate holder 608 by introducing SiH.sub.4 gas through the gas feed pipe 605 into the plasma generation chamber 601, applying a microwave energy through the microwave introducing window 603 into the plasma generation space while effecting a magnetic field in the plasma generation space by the magnet 609 and applying a D.C. bias voltage between the grid electrode 611 and the conductive substrate holder 608 by the D.C. power source 600 to cause an ECR type microwave discharge of generating plasma which is followed to pass through the grid electrode into the film-forming space of the deposition chamber 602, whereby a polycrystalline silicon film is formed on the single crystal silicon wafer 613 maintained at 200.degree. C. Likewise, the foregoing literature discloses the formation of a crystal silicon film by causing homoepitaxial growth on a single crystal silicon wafer maintained at 400.degree. C.
The inventors of the present invention have used a commercially available glass plate (trade name: No. 7059 glass plate, product by Corning Glass Works) instead of the foregoing single crystal silicon wafer 613 and have tried to form a polycrystal silicon film on said glass plate maintained at 400.degree. C. by repeating the film-forming procedures described in the foregoing literature. As a result, it has been found that a practically acceptable polycrystalline silicon film is rarely formed on the insulating substrate (glass plate).
The reason why a practically acceptable polycrystalline silicon film could not be obtained in this case is considered due to that a D.C. bias voltage was not effectively applied between the grid electrode 611 and the substrate 613 because said substrate was insulating. Another factor considered is the reason that lattice matching was not effected for a film to be formed during its formation because the substrate 613 was not a single crystal silicon wafer but an insulating glass plate.
In view of the above, it is difficult to stably and repeatedly form a high quality polycrystalline semi-conductor film of large area on an insulating substrate such as a glass plate which is desirably usable in a TFT by any of the known plasma CVD film-forming methods.
Now, there has been proposed a TFT having a semiconductor layer comprised of a polycrystalline silicon film to be used in a liquid display of active matrix system (this TFT will be hereinafter referred to as "active matrix polycrystalline silicon TFT"). This active matrix polycrystalline silicon TFT is generally prepared in the following way. That is, a transparent electrode comprising an ITO film is formed on a high quality insulating transparent substrate such as a quartz glass plate and thereafter, a polycrystalline silicon film to be the semiconductor layer is formed on said transparent electrode. When the LP-CVD method (low pressure chemical vapor deposition method), which is considered as being effective for the formation of a high quality polycrystalline silicon film, is employed for the formation of said semiconductor layer, silane gas is used as the film-forming raw material gas and film-formation is carried out at a film deposition temperature of 700.degree. C. or more. During the film formation, said silane gas is decomposed to generate hydrogen radicals which unavoidably occur at the ITO film being maintained at elevated temperature, where the hydrogen radicals react with the constituent oxygen atoms of the ITO film to make the ITO film opaque. In this case, the opaque ITO film is unable to function as the transparent electrode. In consequence, the resulting active matrix polycrystalline silicon TFT becomes such that is not practically usable. In this respect, the LP-CVD method is not practically applicable for the formation of a polycrystalline silicon film to be the semiconductor layer of the active matrix polycrystalline silicon TFT. There is also another problem in the case of forming the semiconductor layer comprising a polycrystalline silicon film of said TFT by the LP-CVD method that since the film formation is carried out at an elevated temperature of 700.degree. C. or more as described above, a less heat-resistant inexpensive material such as soda-lime glass, synthetic resin film, etc. cannot be used as the substrate.
There is a proposal to form a polycrystalline silicon film to be the semiconductor layer of the active matrix polycrystalline silicon TFT by the molecular-beam evaporation method, wherein said film is formed on an insulating transparent substrate at a deposition temperature of 400.degree. C. and under an ultra-high vacuum condition of about 10.sup.-10 Torr with the use of a single crystal silicon or a polycrystal silicon as the evaporation source. And the active matrix polycrystalline silicon TFT obtained is such that has a ON/OFF electric current ratio in the range of 10.sup.3 to 10.sup.4 and a carrier mobility in the range of 2 to 10 cm.sup.2 /V.s which are not practically acceptable (see, THIN FILM HANDBOOK, p. 625, published Dec. 10 of 1983 by KABUSHIKI KAISHA Ohm Sha of Japan).